Class Selector

            CSS:
            <style>
                body { font-style: normal; }

                .opacity { opacity: 0.5; }
            </style>

            HTML:
            <p>Normal font-style taken from the body style</p>

            <p class="opacity">P text with opacity  </p>

            <div class="opacity">DIV text with opacity</div>
            

Result:

Normal font-style taken from the body style

P text with opacity

Note: the same class may be used by differents tags.